1. Field of the Invention
The present invention relates generally to a method for cleaning molding compound tablets that are used for encapsulating semiconductor device packages, and an apparatus therefor. More particularly, the present invention relates to a method for removing dust and/or other contaminants from molding compound tablets that are used for encapsulating semiconductor device packages and an apparatus therefor.
2. Description of the Related Arts
The integrated circuit manufacturing process basically comprises wafer preparation, wafer fabrication, electric test (die sort) and assembly processes. In the wafer preparation process, a crystal of silicon is first ground to the required diameter, then a designated flat side is ground on, and then individual wafers are sawed from the crystal. Each raw wafer is polished to optical flatness and to the required thickness.
In the wafer fabrication process, the semiconductor device or integrated circuit (IC) is created in and on the wafer surface, which requires that a number of sophisticated processes be performed in a sequential manner. Although the actual number of steps the wafer goes through will vary depending on the type and complexity of the circuit being built during the wafer fabrication process, three basic operations are generally performed on the wafer: layering, where thin layers of different materials are grown on or added to the wafer surface; patterning, where portions of the thin layers are selectively removed from the wafer; and doping, where the resistivity and conductivity type of selected regions in the wafer are changed or altered by the addition of dopants.
After the electrically active regions are created through the doping sequences in the wafer surface, a metallization process is performed whereby the regions or components on and in the wafer surface are wired together. After this metallization process, the circuit or device is operationally complete. However, since the circuit is vulnerable to contamination and the thin fragile metallization layer is susceptible to scratching, the wafer is covered with a layer that acts as a contamination barrier and protects the metal layer.
After the wafer fabrication process, the wafer is separated into individual chips and mounted and connected to a chip package for the electrical die sort test. The good die which pass the tests are attached to a pad of a lead frame, and the chip is electrically connected to leads through wires.
Finally, the chip is encapsulated with a molding compound to give shape to the package body and to provide protection for the electrical interconnections and chip from exterior environmental stresses.
The molding operation is generally carried out using a molding press. Tablets of a molding compound are melted by increasing the temperature of the press, and the hot melted molding compound is injected and then cooled to room temperature to provide a package body. The molding compound is the most widely used plastic packaging material for encapsulating various types of microelectronic devices for commercial applications. Transfer molding is the process used almost exclusively with molding compounds.
A molding compound is a granulated powder that is normally used in the form of a tablet (preform). In a conventional transfer molding process, one molding time cycle takes between 60 seconds and a few minutes. It consumes one to several tablets, which are preheated to a temperature of about 66.degree. C. with a radio frequency (RF) pre-heater until it achieves the consistency of a soft dough. These soft tablets are then used to encapsulate up to several hundred microelectronic devices using the conventional transfer molding press at a temperature of about 150 to 190.degree. C. (300 to 375.degree. F.) and a pressure of about 4.1 to 8.3 Mpa (0.600 to 1.20 ksi).
The tablets are stored until they are required for the molding process. However, the tablets may be stored for a considerable period of time, and during such storage, dusts or motes may contaminate them. These contaminants may in turn cause the formation of voids in the molded package body. As such, when they are introduced into the molding die, they may cause a change in the speed of molding compound that is flowing into the molding die, resulting in a non-homogeneous molded body. Further, the contaminants may accelerate the aging of the molding press and cause erroneous operation of the sophisticated sensors of the automated molding equipment.
The tablet contamination problems described above account for more than 50% of the total tablet-related problems. Accordingly, cleaning of the molding compound tablets is very important for successful encapsulation.
To minimize possible contaminating particles on the tablets, various techniques such as air blowing, brushing, vacuum cleaning or vibrations are employed. All of these methods, however, only detach or drop the dust or other contaminants from the tablets. Therefore, the dusts may still fly or float in the air, and must later be removed by a vacuum cleaner after being collected. The flying dusts may contaminate the just cleaned wafer, especially when the vacuum system does not remove all the flying dusts.
FIG. 2 is a schematic diagram of conventional cleaning system for removing dusts or other contaminants from the molding compound tablets.
With reference to FIG. 2, the conventional cleaning system comprises a tablet container 14, e.g., a tablet hopper carrying molding compound tablets 18, and parts feeder 10, to which the tablets 18 are loaded from the container 14. Vacuum sweeper 16 is provided above the container 14 and the parts feeder 10. When a tablet 18 in the container 14 is loaded to the parts feeder 10, the vacuum sweeper 16 starts operating to suction the dusts or contaminants from the tablet 18.
The cleaned tablet 18 is transferred to transfer line 12, which moves the tablet 18 to molding equipment 20. The transfer line 12 may be provided with a vibrator 13 to vibrate the tablet 18 so as to detach contaminants from the tablet 18.
However, the dislodged dusts or contaminants are not necessarily removed from the tablet and they may still fly or float in the air around the tablet, and thus contaminate the tablet.